I can't paint in the UV map editor
For viewport painting: 

make sure you have selected the correct image in the slot option ( left side panel slot tab)

uv editor:

make sure you have the texture active by selecting it from the drop down menu in your uv editor.

For both paint modes : 

make sure your current texture brush isn't the target texture you're trying to paint on. In the texture option for your brush set it to none ( double check and make sure your mask doesn't have the same texture either )